A double flywheel of the above kind is described in the specification of U.S. Pat. No. 4,903,544 and the corresponding French published patent application No. FR 2 626 336A. In those documents, the viscous damping means is arranged radially inward of the resilient means, being mechanically coupled between the two masses. The viscous damping means includes a sealed cavity which is at least partially filled with a fluid, and is mounted axially on a first one of the two said masses. This viscous damping means is of dimensions which are chosen to suit the application to which the flywheel is to be put, in order to obtain a predetermined degree of damping.
The viscous damping means also includes two closure members or cover members which are fixed to a spacer member, with these three members together defining the cavity of the damping means. The closure members are joined to each other, and are fixed, at least in rotation and optionally with a clearance, to a damper plate or damping wheel. The latter is arranged to come into engagement with the series of resilient means already mentioned, the latter being operatively interposed between the two masses. The double flywheel includes a first part (which will be referred to as a first damping control means), which is mounted in rotation on the first mass, and which is coupled by the said series of resilient means to the damper wheel. That series of resilient means is mounted in effective engagement with a radial abutment surface which is in a fixed position with respect to the first mass.
It follows that the said resilient means have an effective stiffness which varies under the action of centrifugal force, so that when the engine of the vehicle is running at high speeds, they function imperfectly, and may even become jammed against the radial abutment surface. This is detrimental to the performance of the double flywheel, and also to good absorption of vibrations, so that satisfactory damping is thus not obtained.
In addition, the viscous damping means, which serves its purpose during starting and stopping of the engine, continues to have effect at high engine speeds, to the detriment of good damping.